Unwanted capacitance at the connection between signal conductors and circuit elements has long been a phenomenon with which circuit designers have had to contend. Generally termed "parasitic" capacitance, it is commonly manifested as a capacitive effect between signal leads and the metal package of a circuit element, and as a capacitive effect across a circuit node caused by the disposition of two conductors in close proximity. Because capacitors can pass alternating current (AC) signals, parasitic capacitance of sufficient magnitude can both weaken signals and couple them into circuits where they are unwanted. The effects of parasitic capacitance cause signal degradation and limit the upper frequency at which a circuit can be operated.
Thin film device technology has reduced the phenomenon with respect to the capacitance which existed between input and output leads and element packages in a circuit node. But a residual parasitic effect remains in nodes where a thin film circuit element is connected between two microstrip conductors. The edges of the microstrip conductors nearest to the thin-film circuit element act as capacitive plates because the microstrip conductors are metal bodies of finite surface area separated by a small increment of distance across dielectric substances, i.e., air and the substrate upon which the conductors and the circuit element are deposited. The result is parasitic capacitance between the microstrip conductors and in parallel with the thin-film circuit element in the node.
In circuits where the thin-film circuit element and/or the conductor elements in the circuit node are superconductors, the combined effects of high signal frequencies and physically small circuit elements magnify the problem. Superconducting thin-film circuit elements are often non-linear devices whose resistive characteristics are controlled by varying bias currents passing through them. Parasitic capacitance is of little consequence when the superconducting circuit element is in its lowest resistance (i.e., superconducting) state, but becomes more significant and harmful to performance as the circuit element presents higher resistance to signals. In microwave and millimeter wave circuit nodes, parasitic capacitance is increasingly critical because of the inverse relation of impedance and frequency for capacitive circuits. Undesirable signal coupling across the circuit element connection is more pronounced and damaging at progressively higher frequencies.
The negative effects of parasitic capacitance are particularly significant in high frequency superconducting circuits. Because the input and output conductor segments are often superconductors, which by definition exhibit extremely low resistance, any capacitive coupling across an interface between conductor lines and a superconducting circuit element can cause serious signal degradation of leakage, depending on the application. Additionally, the physical length of superconducting circuit elements at millimeter wave frequencies can be on the order of 5 millimeters and less, making for very small dielectric gaps between the ends of the conductor elements between which the superconducting circuit element is connected. Thus, it is desirable to take measures to minimize the capacitance caused by the physical proximity of the input and output superconductors in a superconducting circuit node.